Acetazolamide has been demonstrated to produce vasoconstriction and to decrease the rate of aqueous humor formation in enucleated, arterially perfused cat eyes. These responses of acetazolamide can be abolished by the concomitant administration of phencyclidine. Experiments performed in vivo utilizing rhesus monkeys, indicate that phencyclide can also block the aqueous humor actions of acetazolamide in this living species. Since phencyclidine does not influence the acetazolamide-carbonic anhydrase interaction, it must be concluded that the inhibitory effects of phencyclidine are probably mediated by its observed action on the vasculature of the eye. Dopamine, hydroxyamphetamine and phenylephrine have been found to decrease aqueous humor production in the enucleated cat eye preparation. Data obtained indicate that the response is mediated by activation of afferent ciliary process blood vessels.